This invention relates to a load-bearing non-pneumatic tire ("NPT" for brevity) having a trapezoidal cross-section and angularly oriented, oppositely directed planar rib members (referred to as "crossed ribs") integrally connected by a web member which makes the NPT non-expansible. By "non-expansible" we refer to the diameter of a wheel fitted with a NPT, which diameter does not get larger when the wheel is rotated, due to the centrifugal forces generated during operation.
The trapezoidal non-pneumatic tire ("TNPT" for brevity) is so configured from an elastomeric synthetic resinous material having specified desirable properties, as to provide optimum handling, cornering and load bearing characteristics for a given mass of resinous material. The term "handling" is used to define the general responsiveness of the vehicle to the expectations of the driver, but more specifically refers to the dynamics of the tires on the vehicle, and in turn, the vehicle itself, due to lateral acceleration. By "elastomeric synthetic resinous material" or "elastomer" we refer to a stiff, resilient, material having specific characteristics defined hereinafter. Rubber, whether natural or synthetic (particularly, styrene-butadiene rubber, SBR), and blends thereof, no matter how blended or vulcanized, is not an elastomer as defined herein, as it fails to meet the criteria set forth herebelow.
In particular, the TNPT provides better handling and cornering than the rectangular non-pneumatic tire ("RNPT") disclosed in the parent case. Either NPT (as used herein the acronym NPT refers to either a RNPT or a TNPT, or both) allows itself to be deformed due to compression, and reverts to its original cylindrical shape while carrying its share of a load, typically as a tire mounted on a wheel rim of an automobile or a motorcycle. The dynamics of such deformation under load determines whether handling is satisfactory or not, such judgment generally being made with respect to a conventional pneumatic tire. Of course, a pneumatic tire has sidewalls which are smoothly and continuously blended into the side edges of its tread.
When the tire of a pneumatic tire is deflected laterally in the inboard direction, the portion of the sidewall nearest the tread comes into contact with the road. By "inboard direction" we refer to the direction in which the vehicle is being turned. With greater deflection, as in a tighter turn at higher speed, progressively more of the sidewall contacts the road. Under the severest conditions, the identifying lettering on the sidewall may be scuffed away by abrasion with the road surface.
Though one might reasonably expect a RNPT to provide optimum handling under lateral acceleration, because of the overall support provided by a rectangular configuration, a RNPT exhibits marginally satisfactory handling when it is used under conditions which generate a high lateral acceleration. Under such dynamic conditions, the entire annular portion of the RNPT in contact with the road, is deflected laterally without benefit of any restraint by its sidewalls, because the RNPT doesn't have any. It will be recognized that the tread of a NPT comes to an abrupt end at each of its shoulders. Under severe cornering conditions, the tread edges of the RNPT are severely abraded.
The unique coaction of the structural elements of the TNPT, by virtue of the distribution of forces due to its trapezoidal shape, provide the TNPT with essentially the same or better deformation and load-bearing characteristics than those of a RNPT having the same mass. Though the ride provided by each belies the stiffness of the elastomeric material and the total lack of air trapped within, the trapezoidal shape of its cross-section provides a unique handling advantage over both a RNPT and a pneumatic tire on a passenger automobile, yet with no substantially noticeable loss of the high degree of comfort provided by the pneumatic tire.
As in a RNPT, the unique open construction of the TNPT dissipates heat generated during continuous cycling between alternating compressive and tensile forces in play during operation. Like the RNPT, the TNPT is ideal for a wide spectrum of wheeled vehicles in which (i) the relatively smaller space occupied by the NPT is desirable, (ii) the susceptibility to puncture is obviated, and (iii) failure of the NPT on special-purpose vehicles such as a personnel carrier due to sudden and serious damage, as for example a bullet, is progressive, rather than sudden, and the effect of such damage is thus minimized.
The overriding advantage of a pneumatic tire is the cushioning it provides, which cushioning, to date, as far as we know, has not been effectively mimiced with a NPT. Its disadvantage is its susceptibility to being punctured. We know of no construction for an elastomeric NPT, other than a RNPT or TNPT, which is reliable and rugged enough to withstand the rigors of normal operation when mounted on the wheel of an automobile.
As will readily be realized, NPTs have been routinely used on vehicles where neither handling, nor cushioning the load is a prime consideration, as for example in fork lift trucks, carts, wheelbarrows, tricycyles, and the like. Even so, the use of NPTs in such applications was less than satisfactory because prior art NPTs had undesirable handling characteristics. In addition, one could not provide a variable spring rate in a prior art NPT without changing either its design, that is, its structural configuration, or the materials of its construction. Solid tires, in particular those made from vulcanized rubber, were subject to high heat buildup and subsequent degradation after only constricted usage over a severely limited period. No prior art NPT that we know of suggests an integral NPT with a central web and crossed ribs which are able to deform locally, that is, near the point where the NPT contacts the surface on which it is run, yet discharges its load-bearing and cushioning functions in a manner analogous to that in which a pneumatic tire discharges such functions.
In U.S. Pat. No. 3,188,775 to Cosmos there is disclosed a rubber wheel, he refers to as an expansible sleeve holder, over which an abrasive belt is trained. The wheel is provided with an expansible structure having circumferentially spaced-apart webs in planes at an angle to that passing through the axis of rotation of the wheel, the webs extending radially in a common direction. The angulated webs in the Cosmos wheel, viewed as rib members (ribs), are connected by a planar central web illustrated in the drawing (FIG. 2 of the '775 patent) but not otherwise referred to because the web is an incident of the manner in which the wheel is molded. The ribs provide a specific function, namely to permit the peripheral rim of the wheel to expand due to centrifugal force under which the normally concave (`dished`) circumferential surface of the wheel becomes a cylindrical surface.
The open construction of our TNPT not only serves to cool it during operation, but permits an economy of highpriced elastomer. Of course, numerous prior art configurations of tires have sought to economize on material, as for example, illustrated in U.S. Pat. Nos. 1,441,654 and 1,493,923 to Austin, and Deister respectively, without providing the operational characteristics under high lateral acceleration, of our TNPT.
The concept of providing a web in a solid rubber tire is also old, having been taught in German Offenlegungsschrift No. 24 60 051. For additional strength, the upper and lower flanges of the I-beam-like member are connected with interdigitated sinusoidal scallops along its inner surface. This manner of strengthening the structure is quite different from the oppositely directed ribs on either side of the web of the NPT. This reference, and all prior art references failed to recognize the peculiar function of planar oppositely directed angulated ribs which are undercut to help ensure that bending is negated, so that, instead of bending, they will buckle when the critical load is exceeded. It is this peculiar characteristic of compressive deformation of the TNPT in normal use, coupled with buckling of the ribs when the critical load is exceeded, which is instrumental in giving the TNPT `ride` characteristics which mimic those of a conventional pneumatic tire, and handling characteristics which may improve them.